Converting yeasted sandwich loaf to sourdough loaf questions

Hello, for the first time I am attempting to convert a favorite yeasted sandwich loaf recipe to a sourdough recipe. If possible, any pointers are welcome. That being said, I do have a few specific questions.

How much starter would you use for 1000 g of flour? I found a recipe (on this site) that calls for 300g of starter for a loaf with 500g of addition flour in it. That seems like way too much! I previously have been making 1000g of dough + 200g of starter.